This invention relates to an oil system, in particular a hydraulic system or a lubricating oil system, having the features of the preamble of claim 1. This invention also relates to a filter for such an oil system as well as the use of a turbidity sensor in such an oil system.
Hydraulic oils and lubricating oils should be designed to be environmentally friendly, in particular biodegradable, for reasons of environmental safety. These conditions lead to oils having a comparatively low stability with respect to hydrolytic cleavage in contact with water. Therefore, in the case of oils, in particular rapidly biodegradable oils, but also traditional mineral oils, a low water content makes a considerable contribution to a long oil standing time. An unacceptably high water content can lead to aging products, especially by way of hydrolysis, and can cause material problems and function problems in the equipment supplied or working with these oils. Likewise, serious damage or functional disorders may be caused in hydraulic systems due to thermal decomposition of the oil, due to dissolution or due to chemical attack on components. Such damage can be prevented by a sensor which detects the impurity content in oil, e.g., by shutting down the oil system promptly.
British Patent 2,194,333 A describes a sensor for monitoring the impurity content in an oil. The sensor senses a value which correlates with the impurity content and is configured as a turbidity sensor which measures the turbidity of the oil which occurs as a function of the impurity content. To this end, the oil to be monitored, or at least a partial stream thereof, is passed through a measuring distance in which light is beamed into the oil to be monitored. The light transmitted by the oil in the measuring distance is measured and a signal value that correlates with the impurity content is generated as a function of the measured intensity of the transmitted light, and this signal value is used to monitor the impurity content in the oil.
U.S. Pat. No. 4,499,376 discloses a device for measuring a particle impurity in a liquid, whereby the measurements are based on the permeability of the liquid for infrared light. Accordingly, an infrared sensor measures a turbidity of the liquid which occurs as a function of the impurity content and it generates electric signals that correlate with it.
U.S. Pat. No. 5,599,460 describes a filter for an oil system which contains in a housing a filter element and a sensor for monitoring the impurity content in the oil. The sensor is situated upstream from the filter element in the housing.
In their article xe2x80x9cPerspectives for environmentally friendly hydraulics,xe2x80x9d printed on pages 352 through 367 of the journal xe2x80x9co+p xc3x96lhydraulik und Pneumatikxe2x80x9d [Oil Hydraulics and Pneumatics], vol. 41 (1997) no. 5, on page 359, Kempermann, Remmelmann and Werner present an oil system in which a hydraulic pump connected to a reservoir on the intake side supplies oil to a feeder line on the pressure side. This feeder line carries the oil to a hydraulic system that is driven or operates with the oil. Downstream from this hydraulic system, the oil is sent back to a reservoir. Downstream from the hydraulic pump and upstream from the hydraulic system, a bypass line is connected to the feeder line; an electromagnetically switchable valve is situated in this bypass line and downstream from this valve there is a water-absorbent filter element. The bypass line also opens into a reservoir, bypassing the hydraulic system. Upstream from the hydraulic system, a sensor is arranged in the feeder line upstream from the bypass connection; this sensor monitors the water content in the oil and detects when it exceeds a saturation limit.
The sensor used for this purpose determines the increase in flow resistance of a pilot stream through a layer of water-absorbent starch polymers. If the pressure medium occurs, i.e., the hydraulic oil, becomes oversaturated, the polymer swells due to uptake of water. The elevated pressure acts on a differential pressure switch which thus delivers the desired warning signal. To this extent, this sensor senses a value which correlates with the water content of the oil.
The sensor is connected to the above-mentioned valve, which blocks the bypass in a first switch position and opens it in a second switch position. As soon as the sensor senses that the saturation limit has been reached, the sensor switches the valve, thus opening the bypass, so that the oil contaminated with an elevated water content flows through the filter element. In this way, the unwanted water content of the oil can be reduced.
In xe2x80x9cMSR Magazinexe2x80x9d 1-2/199, pages 10 and 11, Lauri Tuomaala reports on methods of measuring the moisture content or the water content of the oil in a lubrication system of a paper machine. By monitoring the water content of the oil in the lubrication system in combination with corresponding measures to reduce the water content of the oil, the maintenance costs of an installation equipped with this lubrication system can be greatly reduced and the useful life of the installation can be prolonged. To monitor the water content, a measuring transducer or sensor is used, measuring the water content of the oil on the basis of the water activity. The water activity correlates with the water content of the oil.
In order for a washing machine for washing clothing items to have the lowest possible water consumption for ecological reasons, modern washing machines may be equipped with a turbidity sensor which measures the degree of soiling of the wash water. As long as the degree of soiling of the wash water used for cleaning items of clothing remains below a certain threshold value, the wash water need not be replaced by fresh water. In this way, the water consumption of this washing machine depends on the degree of soiling of the laundry washed with it.
The present invention is concerned with the problem of providing expedient options for an oil system of the type defined in the preamble that will permit economical monitoring of the impurity content in oil.
This problem is solved according to this invention by an oil system having the features of claim 1.
This invention is based on the general idea of using an essentially known turbidity sensor, which operates on an optical principle, for monitoring the impurity content in oil. In doing so, this invention is making use of the finding that the impurity content in oil also causes a visually detectable turbidity which correlates with the impurity content of the oil. This finding is surprising at least inasmuch as oil, in comparison with water, is relatively impermeable for visible light. It was surprising to discover that water, whose light permeability is usually greater than that of oil, can cause turbidity in oil under the specific conditions prevailing in an oil system. Liquids, in particular water, lead to the development of finely distributed droplets in the oil or form a type of emulsion under the operating conditions of an oil system (oil pressure, flow conditions, turbulence). Optical effects in particular, such as refraction and movement of light at the media boundaries, then result in turbidity of the oil.
The term xe2x80x9cturbidityxe2x80x9d is understood here to refer to a reduced permeability or transmission of the oil for beams of light, in particular for infrared light. Since the light permeability of water and oil differ greatly, especially in the infrared range, an increasing water content causes a decrease in the transmission of infrared light and thus an increase in the turbidity of the oil.
Since such turbidity sensors are essentially known and are available commercially at low cost, this yields a price advantage for the oil system according to this invention. In addition, experiments have shown that the turbidity sensor operates relatively inaccurately and responds even at a relatively low impurity content or water content in the oil.
According to a preferred embodiment, the turbidity sensor may be situated upstream from a switching valve in a feeder line, the switching valve in a first switch position supplying the oil to an equipment arrangement that operates with and/or is supplied with oil through the feeder line, and in a second switch position supplying the oil into a bypass line where an element suitable for reducing the impurity content of the oil flowing through it is situated, whereby the turbidity sensor or a control unit communicating with the turbidity sensor actuates the switching valve. Due to this arrangement, the turbidity sensor can cause the switching valve to switch to the bypass line as soon as a threshold value for the impurity content is reached. On the other hand, this arrangement also makes it possible to switch the switching valve back for supplying the equipment arrangement as soon as the impurity content drops back to an allowed range again.
The problem on which the invention is based is also solved by a filter having the features of claim 8. By integration of the sensor and the element which reduces impurities into a filter housing and by arranging the sensor inside the housing upstream from a filter element, this yields an especially compact design which also guarantees that the filter element does not have a negative effect on the measurement of the impurity content in oil.
The problem on which the invention is based is also solved by a method having the features of claim 9. This method makes use of the finding that under the conditions prevailing in an oil system, light, especially infrared light, passes through the oil essentially unhindered while it is absorbed or scattered by water to varying degrees. Accordingly, the transmission of light through the oil changes as a function of the water content. By analysis of the transmission; the water content of the oil can be monitored, and through corresponding actuation of the switching valve, the water content may optionally be reduced.
The problem on which this invention is based is ultimately solved by using a turbidity sensor according to the features of claim 12. This makes use of the finding that the sensor, which is designed for detecting turbidity in water, can essentially also be used to detect an impurity content, in particular a water content, in oil under the operating conditions of an oil system, especially if the sensor operates with infrared light. Since such a turbidity sensor is known per se, monitoring of the water content of the oil of an oil system can be implemented in an especially economical manner.
Other important features and advantages of the device according to this invention are derived from the subclaims, from the drawings and from the respective description of the figures on the basis of the drawings.